Processes for the production of polyalkylene glycol ethers are known. Conventional processes for the production of polyethers are based on the polymerization of epoxides on their own or on the addition of these epoxides with starter components containing reactive hydrogen atoms. Preferred starting components in conventional processes include saccharose (German Auslegeschriften No. 1,064,938 and No. 1,176,358; German Offenlegungsschrift No. 1,443,022), sorbitol (British Pat. No. 876,496, Belgian Pat. No. 582,076 and Modern Plastics, May 1959, pages 151-154) and various difunctional and trifunctional polyhydric alcohols, such as ethylene glycol, propylene glycol, trimethylol propane or glycerol.
Polyether polyols having a hydroxyl functionality of 6 or 8 are obtained by reacting saccharose or sorbitol (or other hexahydric sugar alcohols). Providing they have relatively low molecular weight, these highly functional polyethers are particularly suitable for the production of rigid and semi-rigid polyurethane foams which have good dimensional stability.
For the reaction of saccharose and sorbitol with alkylene oxides on a commercial scale, it is essential that the reaction mixture may be satisfactorily stirred. The intense heating effect which occurs during the reaction of alkylene oxides with hydroxyl compounds may only be adequately dissipated if the reaction mixture is vigorously stirred.
However, mixtures of alkylene oxides with saccharose or sorbitol cannot be satisfactorily stirred under the conditions applied in the commercial production of polyethers, i.e. temperatures of from 95.degree. to 115.degree. C. and pressures of from 0.5 to 3.5 atmospheres gauge. The problem of stirrability particularly occurs in the case of saccharose at the beginning of the alkylene oxide addition when large quantities of unreacted, solid starter material are still present. Inadequately stirrable mixtures of saccharose and alkali metal hydroxide, which is generally used as catalyst in the production of polyethers, may give rise to caramelization and carbonization reactions on the walls of the reaction vessel which inevitably become hot when the reaction mixture is heated. Mixtures of sorbitol and alkylene oxides are also very difficult to stir in the presence of large quantities of unreacted sorbitol, because sorbitol is still present as a solid or just begins to melt at the reaction temperatures (m.p. 97.7.degree. C.). The melts obtained are relatively highly viscous.
Overheating in sorbitol melts, which may easily occur in inadequately stirred reaction mixtures, may give rise in the presence of alkali metal hydroxides to the formation of so-called "sorbitol anhydrides", known as "sorbitans", which in turn results in a loss of functionality in the resulting polyethers. This results in deterioration of the properties of the rigid polyurethane foams produced therefrom.
In order to obviate these disadvantages, it has been proposed to use mixtures of saccharose or sorbitol with low viscosity difunctional or trifunctional polyhydric alcohols as starting components (German Auslegeschrift No. 1,285,741; German Offenlegesschriften Nos. 2,443,372; 2,241,242; 2,521,739 and 2,549,449) or aqueous solutions of the more highly functional starters.
However, the reaction of saccharose or sorbitol with alkylene oxide in aqueous solution or in admixture with glycols is accompanied by undesirable secondary reactions, for example partial hydrolysis of the alkylene oxide by the water used as reaction medium. The hydrolyzed alkylene oxide, the polyalkylene glycols formed therefrom by reaction with more alkylene oxide, and the other secondary products formed (whose presence is reflected in pronounced darkening in the color of the reaction mixture), adversely affect the properties of the rigid and semi-rigid polyurethane foams produced from these saccharose or sorbitol hydroxyalkyl ethers.
One disadvantage of the rigid polyurethane foams produced from saccharose polyethers produced in this way is their often small proportion of closed cells and their resulting poor heat insulating capacity.
In addition, the high proportion of bifunctional and trifunctional secondary products in polyethers of this type means that the rigid polyurethane foams produced from these polyether mixtures do not show significantly reduced dimensional stability.
Polyether polyols which have been obtained by reacting saccharose and/or saccharose/glycol mixtures and which have average molecular weights of from 500 to 1500 are relatively high-viscosity liquids. On account of their high viscosity, the fluidity of the final reaction mixture is reduced during the foaming process. This results in the inadequate filling of molds in the case of molded foams. In addition, there is an unequal distribution in density in the polyurethane foam, resulting in a reduction in compression strength.
Polyethers which are suitable for the production of flexible polyurethane foams are generally produced by known methods by reacting trifunctional polyols, such as glycerol or trimethylol propane, with propylene oxide or ethylene oxide or with a mixture of propylene oxide and ethylene oxide. In many cases, the starter component is also initially reacted with propylene oxide and then with ethylene oxide, resulting in the formation of polyethers predominantly containing primary terminal hydroxyl groups.
However, polyurethane foams produced from such polyether polyols are frequently unable to satisfy the demands with regard to compression hardness. Accordingly, in order to obtain flexible polyurethane foams showing increased compression hardness, it has been proposed to mix bifunctional and trifunctional starters with sorbitol or saccharose and to react these mixtures with a large excess of ethylene oxide to form polyether polyols having an average molecular weight of from 1000 to 10,000 (German Offenlegungsschriften Nos. 2,521,739 and 2,549,449). The reaction of sorbitol alone with alkylene oxides to form relatively high molecular weight polyether polyols having a hydroxyl number of from 20 to 60 is also known.
However, in the production of such polyether polyols by conventional processes, difficulties also arise because the mixtures of the starting components either have a paste-like consistency or are liquids of relatively high viscosity at room temperature or moderately elevated temperature. For this reason, starting components of this type cannot readily be pumped through pipes. This necessitates the use of elaborate apparatus when the polyether polyols are produced on a commercial scale.
It is also not readily possible to satisfactorily stir these mixtures vigorously (as in the case of the rigid foam polyethers as well). For this reason, the reaction velocity of the alkylene oxides is reduced, giving rise to poor volume-time yields in the production of the polyether polyols. In addition, secondary products, which are formed by decomposition of the inadequately stirred reaction mixtures on the hot walls of the reaction vessel, lead to polyether polyols with lower hydroxyl functionality then desired. In many cases, yellow to brown-colored polyethers are obtained.
Accordingly, there is a need for a process for producing polyalkylene glycol ethers by which it is readily possible to produce polyether polyols without the unfavorable properties referred to above, produce polyols with the envisioned functionality, and at the same time, largely avoid the disadvantages of conventional processes.
According to an earlier proposal (German Offenlegungsschrift No. 26 39 083), polyether polyols having an average molecular weight of from 200 to 10,000 and an average hydroxyl functionality of from 2.0 to 7.0 are produced by reacting one or more alkylene oxides, optionally successively, with a mixture of polyhydric alcohols which has been produced by the auto-condensation of formaldehyde hydrate. The auto-condensation is followed by reduction of the condensation products and the optional mixing with additional dihydric and/or trihydric alcohols and/or monoamines or polyamines (the mixture of polyhydric alcohols, hydroxy aldehydes and hydroxy ketones produced by the auto-condensation of formaldehyde hydrate will be referred to hereinafter as "formose" and the polyol mixture produced therefrom by hydrogenation as "formitol").